For Jane Nicklinson, love is now a matter of life and death. Despite proving beyond doubt her devotion to her husband, Tony, the former nurse has agreed to kill him. Five and a half years ago he suffered a catastrophic stroke which left him paralysed from the neck down, able to move only his head and eyes. Within months, realising he was never going to get better, Tony, 56, made it clear to Jane that he wanted to die. What has unfolded since is, to some, the ultimate love story while, to others, it is a plan for premeditated murder.

Speaking exclusively to The Independent on Sunday this weekend, Jane said she wasn't surprised by the request. "It was really upsetting at first but it wasn't a surprise. It's the person that he is." Calmly wiping away the tears of the man she married 24 years ago, she adds: "Life like this is not enough for him, I always knew that; our girls knew it as well.

"It's because I do love him that I would do it. It's his life and it's what he wants, so it should be no one else's business what he does with his life."

The couple were among the first witnesses to appear before the Commission for Assisted Dying, which last week began a 12-month inquiry into the controversial issue – paid for by author and Alzheimer's sufferer Terry Pratchett and the founder of the fashion retailer River Island, Bernard Lewis.

Jane, 55, explains how the stroke left Tony's sharp mind locked inside a broken body – an insufferable fate for the sociable, opinionated engineering executive and rugby enthusiast. Jane told a dozen commissioners, including the former Lord Chancellor, Lord Falconer, and former head of the Metropolitan Police, Lord Blair, that she has tried, unsuccessfully, to convince him that life is worth living and he ought to live to see their two daughters marry and have children.

"I've tried to talk him out of it – we all have – but not any more. He has had a lot of time to think about things and it's what he wants. He's adamant. He should be able to choose," she says.

As the law stands, suicide is legal in the UK, but the only way Tony could physically end his own life is by slowly, and painfully, starving himself to death. If Jane helped him to die, which is what he wants, she would be prosecuted for murder. She would not face prosecution if they travelled to the Swiss clinic Dignitas where at least 150 Britons have so far been legally helped to die. But, on principle, Tony doesn't want to travel to Dignitas: he doesn't see why he should.

So, desperate for the right to choose his own fate, he has launched a legal challenge against Kenneth Clarke, the Secretary of State for Justice. His lawyer, Saimo Chahal of Bindmans solicitors, argues that the law of murder, where consensual killing is absolutely prohibited, disproportionately affects his right to make his own decisions under the Human Rights Act.

If, and it is a huge if, they succeed in changing the law, she will kill him, probably by a fatal dose of medication, here at their Wiltshire home, without fear of being jailed for life for murder. "Until the time comes, I don't know whether I'll be able to do it. I think I will. I hope I will. In an ideal world for Tony, I'd give him a sedative. He'd go to sleep, and then someone else would give him the fatal dose, so I don't have to live with it."

The couple met in 1984 while working in Dubai. He was a civil engineer, she a nurse. For the next 20 years they lived a comfortable expat life in the Middle East, Malaysia and Hong Kong, travelling extensively with their daughters, Lauren, now 24, and Beth, 22.

He was in Athens on business when the stroke, the result of an undiagnosed heart condition, brought all that to a halt. Doctors fought hard to save his life, something that Tony laments, because life as he had known it was destroyed anyway. The Nicklinsons lost their home, their friends, their dogs. They now survive on benefits.

Inevitably, their relationship has changed beyond recognition, from husband and wife to patient and carer. "Tony was always the driving force for everything, the one with all the confidence, and I tottered along behind him. But now he has to sit back and let me do it all," she says.

He weeps throughout the interview. She wipes his eyes, mops his saliva and patiently helps him to communicate using an alphabet board. She appears to have a sixth sense about what he needs: she avoids babying him or making choices on his behalf.

They both know the legal fight will take years. Is life on hold until then? "Life's been on hold for five years," she says. He worries about having another stroke, and surviving; she worries about what will become of him if she dies first. In effect, she has come to terms with her life by giving up on it.

Despite the gravity of their plight, it is the banal routine of daily life that leaves Jane feeling most despondent. There will be no surprises on Christmas Day. Her alarm will go off at 7.15am as usual, and she will get up at 7.20am, to wake Tony for breakfast. The carer arrives an hour later, and so will begin another day. "I do get down, because [life] is just so incredibly tedious."

Using his message board, Tony sums up his plight: "Every day is meaningless." His wife simply agrees: "It's an incredibly lonely life now."